The present invention is used in multilevel coded modulation to be applied to 2.sup.m -value quadrature amplitude modulation (m.gtoreq.4, m: integer). The present invention relates to multilevel code modulation equipment capable of performing correct decoding constantly even if a phase ambiguity exists.
Although the present invention is developed for microwave radio, it can be extensively used in communications other than microwave radio.
Recently, introduction of error-correcting schemes with high correcting capabilities has been advanced in a digital microwave radio system for the purpose of improving the quality of transmission characteristics. Coded modulation has been studied as one of these schemes.
The coded modulation is a combination of mapping based on set partitioning and error-correction coding techniques, and is an error-correcting scheme superior in performance to conventional error-correcting schemes which are independent of modulation/demodulation techniques. Code construction methods are classified into multilevel coded modulation and trellis coded modulation (TCM). The multilevel coded modulation is described in detail in H. Imai, S. Hirakawa, "A New Multilevel Coding Method Using Error-Correction Codes", IEEE Trans. Inf. Theory, vol. IT-23, pp. 371-377. The details of the TCM are described in Ungerboeck, "Trellis-Coded Modulation with Redundant Signal Sets Part 1,2", IEEE Com. Mag., vol. 25, pp. 5-21, Feb. 1987. Since the principle of the error correction is not related directly to the gist of the present invention, a detailed description thereof will be omitted.
It is impossible for quadrature amplitude modulation (QAM) used in digital microwave radio to detect an absolute phase of a carrier on a receiving side, and so a 90.degree. phase ambiguity exists. Generally, differential decoding is used to eliminate the influence of this phase ambiguity. Since, however, errors are doubled by this differential decoding, an error-correcting circuit is placed inside the differential decoding. Therefore, an error-correcting scheme for use in the QAM is required to have characteristics not influenced by this phase ambiguity. Codes meeting this condition are called transparent codes.
A code having the highest correcting capability in the TCM, i.e., a so-called Ungerboeck code is not transparent to 90.degree. and 180.degree. phase ambiguities. The code therefore cannot be correctly decoded except when it is demodulated with the same phase as that on a transmitting side. For this reason, however, an absolute phase can be detected on a receiving side from information obtained by a decoder, and this makes transmission of signals possible without performing differential decoding.
FIG. 6 is a block diagram showing 16QAM for encoding two lower levels, as an example of the TCM. Reference numeral 11 denotes a communication equipment constituted by a transmission unit 500 and a reception unit 600. An input serial signal from a terminal 50 of the transmission unit 500 is distributed to three portions by a first converter 510. In the TCM, unlike in the multilevel coded modulation, both signals of levels 1 and 2 are encoded by a single convolutional encoder 520. Signals of levels 3 and 4 which are distributed to uncoded levels are directly applied together with the signals of levels 1 and 2 which are encoded by the convolutional encoder 520 to a mapping circuit 530. The mapping circuit 530 outputs the coordinates of signal points corresponding to the input signals according to a signal arrangement based on set partitioning shown in FIG. 4. The output from the mapping circuit 530 is applied to a modulator 540 and 16QAM-modulated by the modulator 540. This modulated signal is output to a terminal 60.
The signal received by the reception unit 600 is applied to a terminal 70. A demodulator 610 demodulates the input signal, converting the signal into a digital signal. The output from the demodulator 610 is applied to a phase shifter 620. The phase shifter 620 executes a phase rotation of 0.degree., 90.degree., 180.degree., or 270.degree. and supplies the resultant output to a Viterbi decoder 630 and a decision unit 640. Decoding operations of the levels 1 and 2 are performed by the Viterbi decoder 630, and decisions of the levels 3 and 4 as uncoded levels are performed by the decision unit 640. The decoding and decision results of the individual levels are applied to a second converter 650 and multiplexed into a serial signal. This serial signal is output to a terminal 80. The Viterbi decoder 630 can distinguish a phase difference of 0.degree. between transmitted and received carriers from other phase differences in accordance with metric information used in decoding. Therefore, this information is used to control the phase shifter 620.
Since the TCM described above uses a single convolutional code, a degree of flexibility concerning setting of a maximum coding rate is small. Therefore, the TCM is difficult to use in digital microwave radio in which redundancy must be decreased as small as possible, i.e., a coding rate must be increased as high as possible.
In contrast, the multilevel coded modulation uses different codes at different levels in set partitioning, and so a degree of flexibility of setting a coding rate is larger than that of the TCM. Since the multilevel coded modulation has substantially the same characteristics as the TCM in correcting capability, the multilevel coded modulation can be said to be a coded modulation scheme suitable for the digital microwave radio. The multilevel coded modulation, however, has not been satisfactorily studied yet in its practical applications as compared with the TCM; for example, an application of the multilevel coded modulation to a communication system in which a phase ambiguity exists has not been sufficiently examined yet.